1. Field of the Invention
The present invention relates to an optical module for use in optical communication, and more particularly, to an optical module that is electrically connected through a flexible printed circuit to a circuit board for driving the optical module.
2. Description of the Related Art
A conventional optical module includes a package that accommodates a carrier on which an optical device is mounted, an optical connector that is disposed on one end of the package, a feedthrough that is disposed on the other end of the package, and a flexible printed circuit that is fixed to the feedthrough (for example, see JP 2007-71980 A).
The feedthrough is formed of laminated ceramic, and has electric terminals (radio-frequency terminal and bias terminal) which are disposed on an outer surface of the package perpendicularly to an optical axis of input/output light to/from an optical connector, and a carrier connection terminal which is disposed on an inner surface of the package so as to be connected to the electric terminals.
The flexible printed circuit has an end of one surface stuck onto an entire surface of the feedthrough by an anisotropically conductive adhesive. The flexible printed circuit has connection pads disposed at positions that face the electric terminals of the feedthrough. Here, the anisotropically conductive adhesive is thermally cured while being crimped under pressure, to thereby exhibit conductivity only between the surfaces facing each other.
The feedthrough and the flexible printed circuit are allowed to adhere to each other with the anisotropically conductive adhesive so that the electric terminals disposed on the feedthrough and the connection pads disposed on the flexible printed circuit are electrically connected to each other. Consequently, the degree of freedom for the mechanical viewpoint can be secured. Further, the optical module has no member inhibiting a radio-frequency characteristic, such as a lead pin, and hence the radio-frequency characteristic can be also ensured.
However, the conventional technology suffers from the following problems.
In the conventional optical module, when an optical device having a plurality of channels is incorporated into one package, the density of the electric terminals disposed on the feedthrough becomes higher. This leads to a problem that the wiring density of the flexible printed circuit becomes too high to physically arrange the wiring of the flexible printed circuit.
When the wiring density of the flexible printed circuit becomes higher, a radio-frequency line (RF line) that is disposed on the flexible printed circuit and connected to the radio-frequency terminal, and a bias line (DC line) that is similarly connected to the bias terminal cross each other. For this reason, isolation between those lines cannot be ensured, resulting in problems that a signal in an adjacent channel produces crosstalk, and that an output signal crosses over into an input signal to oscillate.
It is conceivable that the flexible printed circuit is configured by a multilayer structure having three or more layers to reduce the wiring density of each layer in the flexible printed circuit. This leads to another problem that the bending property of the flexible printed circuit is deteriorated to reduce the mounting property on the circuit board.